Ship radars transmit radio waves from an antenna which rotates at a predetermined cycle on a horizontal plane, and receives reflection signals from targets in the surroundings from the antenna. Each received signal (reception signal) is converted into reception data by an A/D converter. The reception data is suitably processed and then is displayed on a display unit as a radar image. An operator of the radar can find out the situation regarding the targets in the surroundings by checking the displayed radar image.
A distance r to a target can be obtained based on a time length required for the antenna to receive the reflection signal from the target from its transmission. Moreover, the direction of the target can be obtained based on an orientation θ of the antenna when it received the reflection signal from the target. In other words, it can be said that the signal received by the ship radar indicates a position of the target in a polar coordinate system (r, θ).
There are cases where the reception signal contains clutter (a reflection wave from the sea surface or rain) and noise. If clutter and noise are displayed on the radar image, discriminability of the target on the radar image decreases. Thus, conventionally, in ship radars, a scan correlation for discriminating the target from clutter based on stability of the reception signals between scans and reflecting it on the radar image has been known. Patent Documents 1 to 4 disclose such kind of scan correlation, for example.
Patent Document 1 discloses a configuration in which the coordinates of the reception data indicated in the polar coordinate system are converted into an orthogonal coordinate system and the scan correlation is performed by accessing an image memory by write-and-read addresses corresponding to the coordinates in the orthogonal coordinate system. In other words, in Patent Document 1, the scan correlation is performed by converting the reception data from the polar coordinate system into the orthogonal coordinate system.
In this regard, Patent Documents 2 and 3 point out problems in performing the scan correlation after the conversion of coordinates into the orthogonal coordinate system. For example, Patent Document 2 points out, in the scan correlation in which the conversion from the polar coordinate system into the orthogonal coordinate system is performed, a problem that reception data (echo data) needs to be selected and interpolation data needs to be created in the coordinate conversion, and the original echo data cannot be used in the scan correlation as it is. Moreover, Patent Document 3 points out a problem that the accuracy of the correlation easily degrades since the corresponding relationship between the reception data and the sweep will be lost when the reception data is written in the image memory (memory for correlation) by an address in the orthogonal coordinate system.
Thus, Patent Documents 2 and 3 disclose configurations in which the scan correlation is performed with the polar coordinate system remained. Patent Document 2 claims that a resolution can be improved by performing the scan correlation by using the coordinate system when the signal is received. Moreover, Patent Document 3 claims that the accuracy of the scan correlation does not degrade by keeping the corresponding relationship between a true transmitting direction and a relative transmitting direction of the signal when writing/reading it into the image memory.